Know if the ductwork can carry the new unit.
Enter the duct runs you can see and compare the estimated airflow with the system size, before an undersized return freezes the coil or trips the high limit.
How big a duct does this airflow need?
Set the airflow and friction rate to see the duct size. Results are estimates, not a duct design.
Note: sizes are a simplified estimate based on common residential friction rates. Rectangular equivalents use the Huebscher formula, De = 1.30 × (a × b)0.625 / (a + b)0.25, with width and height in inches.
Round Duct Sizing & CFM Capacity Chart
Standard sheet metal and flexible duct sizing guide at 0.10 in. wg / 100 ft friction loss:
| Duct Diameter | Airflow Range (CFM) | Avg Velocity (FPM) | Typical Residential / Light Commercial Use |
|---|---|---|---|
| 5" | 40 - 65 CFM | 450 FPM | Small bathroom, closet supply |
| 6" | 75 - 110 CFM | 500 FPM | Standard bedroom, small office |
| 7" | 120 - 160 CFM | 550 FPM | Master bedroom, living room branch |
| 8" | 180 - 240 CFM | 600 FPM | Large living area, kitchen supply |
| 9" | 260 - 320 CFM | 650 FPM | Open floor plan branch |
| 10" | 350 - 450 CFM | 700 FPM | Main trunk branch (approx. 1 Ton) |
| 12" | 550 - 700 CFM | 750 FPM | Sub-trunk line (approx. 1.5 - 2 Tons) |
| 14" | 800 - 1,000 CFM | 850 FPM | Main trunk duct (2.5 Tons) |
| 16" | 1,100 - 1,400 CFM | 900 FPM | Central return trunk (3 - 3.5 Tons) |
| 18" | 1,500 - 1,900 CFM | 950 FPM | Large commercial/residential main trunk (4 - 5 Tons) |
Ductwork Is Behind 40%+ of HVAC Breakdowns
Undersized ducts create high static pressure, frozen evaporator coils, and premature compressor burnouts.
Evaporator Coil Icing
Undersized return trunks choke airflow below 350 CFM/ton. Coil temperatures plummet below 32°F, freezing condensate and starving the compressor of heat load.
Furnace High Limit Tripping
Restricted supply plenums trap heat inside furnace heat exchangers. High-limit switches trip within 4 minutes of burner ignition (Carrier Code 33, Goodman 4 Flashes).
ECM Blower Overamp & Burnout
Modern constant-CFM ECM blowers ramp up RPM against high static pressure (> 0.8 in. w.g.), pulling excessive amperage until control modules overheat and fail.
Duct Sizing & CFM FAQ
How do I know if a home's ductwork is undersized for a new AC install?
Measure total external static pressure (TESP) with a dual-port digital manometer. If supply + return static exceeds the manufacturer's maximum rating (typically 0.50 in. w.g. for standard furnaces or 0.70 in. w.g. for air handlers), the ductwork is undersized and will restrict required CFM.
Why do flex ducts need to be sized one inch larger than rigid sheet metal?
Flexible ductwork has internal wire helix ridges and sag that create significantly higher friction loss (equivalent to 0.05–0.08 in. wg/100ft vs. smooth galvanized steel at 0.10). An 8-inch flex duct flows approximately the same CFM as a 7-inch smooth round metal duct.
Is this duct calculator in the ACLogics app?
Yes. Open Tools → Duct Calculator, add each flex, round or rectangular run, and set the system size. If the runs add up to less than the target, it shows May be undersized. Treat that as a reason to measure static pressure, not as proof. See how to use the Duct Calculator.
Field Toolkit
Check the ductwork before you blame the equipment.
Download ACLogics free for iPhone and Android. No credit card needed. Check ducts, look up error codes and diagnose equipment from the same app.